Nuclear fuel assemblies, and in particular fuel assemblies for pressurized-water nuclear reactors, have a bundle of fuel rods which are held in parallel arrangements spaced regularly apart from another by a rigid framework.
The framework of the fuel assembly consists, in particular, of spacer grids which are distributed over the length of the fuel assembly and which define cells for receiving and holding the fuel rods in the bundle.
Guide tubes, which are substituted for some of the fuel rods inside the bundle, pass through some of the cells of the grids, and when appropriate are fixed on the grids which they hold and distribute over the length of the fuel assembly.
The guide tubes are longer than the fuel rods, and protrude from the ends of the fuel rod bundle, at the ends of the fuel assembly. The framework of the fuel assembly is closed by nozzles which are fixed onto the protruding end parts of the guide tubes of the framework.
Fuel assemblies which have a right prismatic shape, generally of square cross-section, are placed in the service position inside the core of the nuclear reactor, so that their longitudinal axis is vertical.
The end nozzle of the fuel assembly which is situated at the lower part of the latter, which is referred to as the bottom nozzle and on which the fuel assembly rests, has supporting legs and an adaptor plate penetrated by openings for fastening the guide tubes and openings for water to pass through.
The top nozzle of the fuel assembly, which is located at the second, upper end of the assembly has in particular a transverse adaptor plate which is penetrated by openings for accommodating and fastening the guide tubes and openings for water to pass through.
In order to improve the performance of nuclear fuel assemblies, it is desired to increase as far as possible the combustion ratio of the fuel contained in the assembly and to improve the hydraulic operating conditions of the assembly, through which a flow of cooling water passes from bottom to top inside the core of the reactor.
In particular, in order to improve the hydraulic performance of fuel assemblies, it is desired to reduce as far as possible the head loss of the cooling water when it passes through the transverse adaptor plates of the nozzles, by trying to obtain the greatest possible transparency, i.e., ratio between the area of the openings and the area of the solid parts of the adaptor plates.
The shape and the distribution of the openings for water to pass through the adaptor plates of the nozzles are defined in such a way as to make it possible to fasten the guide tubes of the assembly (for example twenty-four guide tubes) on the adaptor plates of the nozzles and to limit the axial displacement of the fuel rods of the assembly (for example two hundred and sixty-four rods) by adopting a distribution of the openings, and of the ligament defining the openings, such that each of the rods of the assembly faces a ligament.
In particular, as regards the adaptor plate of the top nozzle of the fuel assembly, the water passage openings are of oblong shape and are interposed between the passage holes for the guide tubes. These oblong openings have lengths matched to the size of the portions of the adaptor plate which are interposed between the guide-tube passages.
Such a network of oblong openings of variable dimensions does not make it possible to obtain a very high degree of transparency for the adaptor plate and a uniform distribution of the openings through which the cooling water passes.